As much importance has been attached to saving in resources and countermeasures to environment in recent years, requirements to reduce fuel consumption of automobiles have been increasingly severe. Automobile tires have also been required to low their rolling resistance so as to contribute to reduction in the fuel consumption. In order to low the rolling resistance of tires, it is generally effective to use a rubber material capable of providing a vulcanized rubber having low heat build-up as a rubber material for the tires.
In general, rubber compositions in which carbon black is compounded into a diene rubber such as natural rubber (NR), polybutadiene (BR), polyisoprene (IR) or styrene-butadiene copolymer rubber (SBR) are widely used as rubber materials for tires. However, the rubber compositions obtained by compounding carbon black into the diene rubber have insufficient heat build-up resistance.
It has heretofore been proposed to use, as a rubber material for tires, a rubber composition with silica compounded in place of carbon black as a reinforcing agent into a diene rubber so as to improve heat build-up resistance. However, the silica-compounded rubber composition has involved problems that since its processability is poorer compared with the carbon black-compounded rubber composition, the heat build-up resistance cannot be sufficiently developed, and that it is insufficient in tensile strength and the like. A cause for these problems is considered to be attributed to the fact that the affinity of the silica for the diene rubber is lower than that of the carbon black, so that the processability becomes poor, and a reinforcing effect cannot be sufficiently developed.
In order to enhance the affinity of the silica for the diene rubber, it has heretofore been proposed to use a silane coupling agent (Japanese Patent Application Laid-Open Nos. 252431/1991 and 252433/1991, etc.). However, the mere use of the silane coupling agent is not yet sufficient for the improving effect on heat build-up resistance, tensile strength and processability.
In order to enhance the affinity of the silica for the diene rubber, it has also been investigated to use a diene rubber with a substituent group having high affinity for silica introduced therein. For example, a diene rubber with a tertiary amino group introduced therein has been proposed for a diene rubber according to an emulsion polymerization process (Japanese Patent Application Laid-Open No. 101344/1989). Besides, a diene rubber with a substituent group having a polar group, such as an alkylsilyl group (Japanese Patent Application Laid-Open No. 188501/1989), a halogenated silyl group (Japanese Patent Application Laid-Open No. 230286/1993) or a substituted amino group (Japanese Patent Application Laid-Open No. 22940/1989) introduced therein has been proposed for a diene rubber according to an anionic polymerization process. However, the diene rubbers with these substituent groups having a polar group introduced therein have demerits that their processability is poor upon blending with silica, and their properties such as heat build-up resistance and tensile strength are not sufficiently improved.
On the other hand, with respect to polyether-based polymers, rubbery or resinous polymers have been known. As examples of rubber compositions comprising a polyether-based polymer as a main component, there have been reported a rubber composition comprising 70 parts by weight of an epichlorohydrin-allyl glycidyl ether copolymer rubber and 30 parts by weight of a low-nitrile rubber, which has excellent paper-feed performance and is useful for platen rolls (Japanese Patent Application Laid-Open No. 261436/1989), a rubber composition comprising 50 to 70 wt. % of epichlorohydrin rubber and 50 to 30 wt. % of butadiene rubber, which has excellent charging property and is suitable for use in charging rollers (Japanese Patent Application Laid-Open No. 164571/1995), a conductive rubber composition comprising epihalohydrin rubber and silica (Japanese Patent Application Laid-Open No. 112653/1987), a rubber composition comprising epichlorohydrin rubber and hydrous silicic acid, which has excellent rancidity resistance and is suitable for use in hoses (Japanese Patent Publication No. 71946/1992), and a rubber composition comprising an epichlorohydrin-propylene oxide copolymer rubber and hydrous silicic acid and having excellent oil resistance, ozone resistance and low-temperature resistance (Japanese Patent Publication No. 62147/1993). Besides, as examples of polyether-based polymers useful as modifiers, there have been reported a copolymer of ethylene oxide and epichlorohydrin, which improves the static electricity-discharging properties of resins such as polyolefins (Japanese Patent Application Laid-Open No. 52986/1991), and an epihalohydrin copolymer rubber which improves the antistatic properties of plastic materials such as ABS resins (Japanese Patent Publication No. 84564/1995). However, these polyether-based polymers have heretofore not been known to have an excellent modifying effect on rubber compositions comprising diene rubber and silica.